Despite the remarkable power and flexibility of human cognition, working memory, the “online” workspace that most cognitive mechanisms depend upon, is surprisingly limited: an average adult human has a capacity to retain only four items at a given time. This capacity is fundamental to cognition: individual variability in capacity is highly correlated with their fluid intelligence and patients with neuropsychiatric disorders often have a reduced capacity. Because it is so basic to cognition, capacity limitations have been well-studied in humans, particularly visual short-term working memory. This has led to several competing theories about the neural basis of capacity limitations. “Discrete” models suggest that capacity limitations reflect a limit in the number of objects that can be simultaneously represented. “Flexible resource” models predict that only the total amount of information available is limited, with information divided among all represented objects. It is also not clear whether the limitation is in stimulus encoding or in maintenance.